1. Field of the Invention
This invention relates to a method for preparing a permanent magnet material of Fe-(Co)-R-B system comprising R which is a rare earth element inclusive of Y throughout the disclosure, Fe, B, and optionally, Co.
2. Prior Art
Typical of high performance rare earth magnets are powder metallurgical Sm-Co base magnets having an energy product of the order of 32 MGOe which have been commercially produced in a mass scale. These magnets, however, undesirably use expensive raw materials Sm and Co. Among the rare earth elements, those elements having a relatively low atomic weight, for example, cerium, praseodymium and neodymium are available in plenty and less expensive compared to samarium. Further Fe is less expensive than Co. Thus R-Fe-B system magnets such as Nd-Fe-B magnets were recently developed as seen from Japanese Patent Application Kokai No. 9852/1985 disclosing rapidly quenched ones.
The rapid quenching process is to inject a metal melt against a surface of a quenching medium for quenching the melt, thereby obtaining the metal in a thin ribbon, thin fragment or powder form. The process is classified into a single roll, twin roll, and disk process depending on the type of quenching medium. Among these rapid quenching processes, the single roll process uses a single chill roll as the quenching medium. An alloy melt is injected through a nozzle against the circumference of the chill roll rotating relative to the nozzle for contacting the melt with the chill roll circumference, thereby quenching the melt from one direction for obtaining a quenched alloy typically in ribbon form. The quenching rate of the alloy is generally controlled by the circumferential speed of the chill roll. The single roll process is widely used because of a reduced number of mechanically controlled components, stable operation, economy, and ease of maintenance
The twin roll process uses a pair of chill rolls between which an alloy melt is interposed for quenching the melt from two opposite directions.
The single roll process results in a quenched alloy in which because the rate of cooling on one surface in contact with the chill roll circumference (to be referred to as roll surface, hereinafter) is higher than the rate of cooling on another surface opposite to the roll surface (to be referred to as free surface, hereinafter) during quenching, the grain diameter near the free surface is larger than the grain diameter near the roll surface by a factor of more than 10, for example.
The twin roll process results in a quenched alloy which does not have a free surface, but has a larger grain diameter near the center of the alloy in a thickness direction since the cooling rate intermediate the opposite roll surfaces is slow.
The thus quenched alloys include a very narrow region having optimum grain diameter and will exhibit high magnetic properties with difficulty.
For this reason, the quenched alloy is ground into a magnet powder including both a fraction of magnet particles having high magnetic properties and a fraction of magnet particles having low magnetic properties. When such magnet powder is dispersed in a resin binder to form bonded magnets, these bonded magnets do not exhibit high magnetic properties as a whole, but have locally varying magnetic properties.